DISEASES OF CULTIVATED PLANTS 333 
upon the transmission of the tobacco mosaic disease. makes the risk 
of transmission from diseased to healthy plants by such handling, 
stand out clearly. ‘This line of transmission was verified on tobacco 
by the writer’s assistant in 1903 (See Bulletin 156, of this Station). 
While the same class of proof for peach yellows is very difficult, 
owing to the latent nature of the disease for some months after first 
infection, the actual results of infection from nearby diseased trees 
make clear the danger of such exposure and the necessity for the 
destruction of diseased trees. Chemical examination of variegated 
or chlorose tissues shows the same compounds, the oxidases, etc., to 
be present and to account for the transformation of the leaf-green 
or chlorophyll, into xanthophyll, or leaf yellow. ‘Thus by degrees 
apparent plant disease mysteries are solved. ‘The weakness of 
variegated plants and their ready susceptibiity to attacks of para- 
sitic fungi are now explained by thisimpaired condition of the leaf 
parts. 
PLANT DISEASES TRANSMITTED IN THE SEED 
The public in general little realizes how many diseases of plants 
are transmitted in the seed, although as the years pass the general 
dissemination of knowledge concerning infection by spores and by 
germs has partly prepared the way. The public mind does not 
longer expect something to grow from nothing. The treatment of 
seed grain, as wheat, oats, barley, etc., todestroy adhering spores 
of the smut fungi, and thus prevent these smuts in the crop, has 
been known for many years. In the early days of the Agricultural 
Experiment Stations, these doctrines and practices in this regard 
were widely disseminated, new impetus being given by the success- 
ful use of hot water following’ the methods of Jensen in Denmark; 
but despite the conquest of the practical control over the order 
Ustiligineae, the smuts, we have only really begun to study the 
matter of seed infecting diseases produced by seed infesting fungi. 
These seed infesting fungi are of two types, viz, first, those whose 
spores adhere to the seed grain as in the case of the smuts of grains 
generally, and second, and more exactly, those fungi which 
develop upon or within the seed largely by their threads or mycel- 
ium, and may, or may not, prevent the germination of the infested 
seed grain. Our knowledge of these strictly seed infesting fungi is 
quite recent; we may point to the work of Prof. Bolley and his 
assistants at the North Dakota Experiment Station, especially upon 
the matter of flax diseases; to the work of Dr. Halsted in New Jersey 
and to Bulletin 173 of this Station by Van Hook. With the tendency 
to continuous growing of flax, in the west there was developed m 
